Exchange 2003 and inter-domain email

Hello all,
Bit of an odd situation here... I work for the US branch of a company based in the UK. We have a domain XYZcorp.com who has a UNIX based mail server. They wil not change to exchange. They want to control the MX record and have all inbound mail go through the UK server. we have an a-host entry us.xyz.com.

The exchange server in the US has its outbound recipeint policy set to:
xyzcorp.com (primary)
us.xyzcorp.com

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